buildworld - Fix breakage

* Fix some _KERNEL_STRUCTURES breakage. This isn't a fantastic solution
but it works.

Reported-by: swildner

Change type of len to size_t.

Return retval if the second sysctl failed, not NULL.

Add the basic of libkcore. Switch pstat to use kcore/kinfo backing,
defaulting to kcore for now.

Add a new system library, libkinfo. It is intended to replace libkvm for
most userland tools, which want to deal with a living kernel, not
post-mortem analysation.

Change the kern.file backing from

Add a new system library, libkinfo. It is intended to replace libkvm for
most userland tools, which want to deal with a living kernel, not
post-mortem analysation.

Change the kern.file backing from a struct file array to a struct kinfo_file
array. The later is an independent structure, which includes the information
useful for userland. This allows changing the kernel-internals without
having to recompile libkinfo and derived programs. This is inspired by the
changes DES did in FreeBSD 5 (struct xfile).

Partly-obtained-from: FreeBSD (kern_descrip.c changes)

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